1. Field of the Invention
The present invention relates to heat exchangers and more particularly, but not by way of limitation, to an in-line exhaust gas heat exchanger for heating or preheating fluid by waste hot exhaust gases and including means for controlling the temperature of the fluid and the temperature of the exhaust gases for prevention of corrosion to the heat exchanger itself.
2. Description of the Prior Art
Heretofore, in-line exhaust heat exchangers of the type disclosed of the patent to Elmendorf, Ser. No. 687,735 dated Dec. 3, 1901 and entilted "Heating Device for Liquids" were used in exhaust gas pipes. It was long ago recognized that such heat exchangers often called economizers are desirable for use in exhaust gas smoke stacks and the like but were extremely limited in their use to non-sulphur or very low sulphur fuels and only on boilers with non-fluctuating loads.
Exhaust gases from oil and coal fired boilers contain sulphur. In an economizer these gases are cooled and a sulphur dew point temperature may be reached. At this dew point temperature, sulphur, together with water vapor condense and form actively corrosive sulphuric and sulphurous acids. This can cause an extremely corrosive situation wherever these liquids do occur.
As stated, this condition previously limited the use of economizers to non-sulphur or very low sulphur fuels. Since boilers with fluctuating loads often caused stack temperature fluctuations resulting in "dew point" situations, in-line exhaust gas heat exchangers were limited to non-fluctuating-load boilers or incinerators.
The use of various alloys in construction of the economizer most often did not stop corrosion and never prevented acid formation. Therefore, a method of controlling the exhaust gas temperature of the economizer is mandatory.
A further problem present in the Elmendorf type device is that even where the damper or butterfly valves for routing the exhaust gases are in the open position a large portion of the hot exhaust gas would still move around the inner exhaust pipe and past the heat exchanger elements, causing unwanted heating of the liquid within the helical chamber.
Further, since the inner pipe of Elmendorf is supported within the outer pipe by means of the helical heat exchanger elements, a great amount of unwanted heat transfer takes place when the damper is open.
Another disadvantage of the prior art devices is that since the outer sleeve or housing is the primary load carrying member, it renders the device very difficult to provide maintenance for the heat exchange elements without first removing the device from the stack and temporarily supporting the stack during such maintenance. Since the economizer is normally inserted in an existing smoke stack, the Elmendorf type device has proved to be incapable of carrying the structural load required without greatly beefing up the outer wall of the heat exchanger or providing some other means for carrying the load of the exhaust stack above the heat exchanger.